GES 430 Environmental Geochemistry Class 6 Acidity, Alkalinity + Buffering

Or 0.25 (K a1 k CO2 P CO2 )^ 2 K a2 √K sp () 1/3 [H + ] = Plugging the numbers in we get a pH of 8.35 Waters will initially be acidic but once they come into contact with dolomite they will become basic. As your book shows calcite has a very similar effects. Interaction with carbonate minerals is one of the best ways of countering the effects of acid precipitation.

If water is in contact with the atmosphere pH calculation known as open system calculations if not they are closed system calculations. Open systems Closed systems For closed systems we often do not know P CO2 must have additional information. One assumption often used is the constant total carbon (C T ) assumption see book page 67.

Acidity is a measure of the ability of a solution to donate H +. Example of “donation” of H + : What will happen OH - is added to a solution containing H 3 SiO 4 - ? H 3 SiO OH - → H 2 SiO H 2 O The solution has donated a H + to the hydroxyl. Alkalinity is the capacity of a solution to accept H +. Example of “acceptance” of H + : What will happen H + is added to a solution containing H 3 SiO 4 - ? H 3 SiO H + → H 4 SiO 4 The solution has “accepted a H +

Quantitatively acidity (designated as C A ) is defined as the sum of the concentration of all species that can donate H + minus the concentration of OH -. Example: write an expression for the acidity of a solution that results from the dissolution of H 4 SiO 4 in pure H 2 O? C A = [H 4 SiO 4 ] + [H 3 SiO 4 - ] + [H + ] – [OH - ] Note that we have ignored the very weak 3 rd and 4 th dissociation of H 4 SiO 4 i.e. we have assumed that H 2 SiO 4 -2 → HSiO H + and HSiO 4 -3 → SiO H + are negligible.

Quantitatively alkalinity (designated as C B ) is defined as the sum of the concentration of all species that can accept H + minus the concentration of H +. Example: write an expression for the alkalinity of a solution that results from the dissolution of H 4 SiO 4 in pure H 2 O? C B = [H 3 SiO 4 - ] + [H 2 SiO 4 -2 ] + [OH - ] – [H + ] Note that we have ignored the very weak 3 rd and 4 th dissociation of H 4 SiO 4 i.e. we have assumed that H 2 SiO 4 -2 → HSiO H + and HSiO 4 -3 → SiO H + are negligible.

Acidity and alkalinity are often given in milli-equivalent per liter (meq/L) = moles/L of the species number of H + that can be donated or accepted by that species. Thus in the examples given above: C A (meq/L) = 2 [H 4 SiO 4 ] + [H 3 SiO 4 - ] + [H + ] – [OH - ] and C B (meq/L) = [H 3 SiO 4 - ] + 2 [H 2 SiO 4 -2 ] + [OH - ] – [H + ] C A = [H 4 SiO 4 ] + [H 3 SiO 4 - ] + [H + ] – [OH - ] C B = [H 3 SiO 4 - ] + [H 2 SiO 4 -2 ] + [OH - ] – [H + ]

Acidity can be determined empirically by titrating a solution with a strong base. Alkalinity can be determined empirically by titrating a solution with a strong acid.

For a solution that begins with a strong acid or strong base the titration curve has an inflection at pH = 7. Around this inflection the curve is nearly a straight line and the addition of relatively little base causes a large change in pH. The amount of base required to reach this inflection is the acidity. Acidity = 5

For a mixture of a weak acid and a strong acid there are several inflection points around which the addition of relatively small amount of base causes a large change in pH. The amount of base required to reach each of these inflection points is defined as a different acidity – total acidity is the total amount of base required to reach the last inflection point. Note that alkalinity is the mirror image of acidity.

Buffering = ability of a solution to resist changes in pH when H + or OH - are added. The buffering capacity or index (B) is defined as B = dC B /dpH or B = dC A /dpH. Relatively large amount of H + or OH - required to change pH B is large. The larger these numbers are the more OH - or H + is required to create the same change in pH. Relatively small amount of H + or OH - required to change pH B is small. Note that B is the slope of curve on an acidity or alkalinity titration curve

To derive B theoretically (book page 80 – 87): 1.Write an equation for C B or C A 2.Differentiate these equations with respect to [H + ] to obtain dC B /d[H + ] or dC A /d[H + ] 3.Use the fact that d[H + ] = 2.3 [H + ] dpH (derived from basic calculus) to convert these to dC B /dpH or dC A /dpH For a system containing a weak diprotic acid: B = 2.3 ( K a1 C A [H + ] (K a1 + [H + ])^ 2 K a2 C A [H + ] (K a2 + [H + ])^ 2 K w [H + ] ) ++ +

B = 2.3 ( K a1 C A [H + ] (K a1 + [H + ])^ 2 K a2 C A [H + ] (K a2 + [H + ])^ 2 K w [H + ] ) ++ + For problem 39 you will need to apply this equation to a solution containing H 4 SiO 4. To do this you will need a value for C A. We derived an expression for C A in this system earlier in the class: Hint on problem set But to use this you need values for [H 4 SiO 4 ] and [H 3 SiO 3 - ] Life will be easier if you use the facts that at pH < 9.83 [H 4 SiO 4 ] >> [H 3 SiO 4 - ] and you can assume that all the dissolved silicic acid is H 4 SiO 4. At a pH of 9.83 [H 4 SiO 4 ] = [H 3 SiO 4 - ] and at pH > 9.83 [H 3 SiO 4 - ] >> [H 4 SiO 4 ] and you can assume that all dissolved silicic acid is H 3 SiO pts extra credit for people who can show why this is true. C A (meq/L) = 2 [H 4 SiO 4 ] + [H 3 SiO 4 - ] + [H + ] – [OH - ]

Buffering capacity of natural systems: The most effective common natural buffer is the carbonic acid – calcite buffer. Clay minerals have a significant theoretical ability to buffer acidic solutions through reactions like: 2KAl 3 Si 3 O 10 (OH) 2 + 2H H ↔ 3 Al 2 Si 2 O 5 (OH) 4 + K + + H 4 SiO 4 muscovite kaolinite aqueous But the kinetics of these reactions are relatively sluggish. Weathering reactions involving quartz, feldspars and ferromagnesium silicates (unweathered igneous rock) are relatively ineffective as buffers.